U.S. patent number 4,329,403 [Application Number 06/257,740] was granted by the patent office on 1982-05-11 for electrolyte-electrode assembly for fuel cells.
This patent grant is currently assigned to Energy Research Corporation. Invention is credited to Bernard S. Baker.
United States Patent |
4,329,403 |
Baker |
May 11, 1982 |
Electrolyte-electrode assembly for fuel cells
Abstract
An electrolyte-electrode assembly for high temperature fuel
cells in which the electrolyte member is adapted to exhibit a more
gradual transition in coefficient of thermal expansion in going
from the anode electrode to the inner electrolyte region and in
going from the cathode electrode to such inner electrolyte
region.
Inventors: |
Baker; Bernard S. (Brookfield
Center, CT) |
Assignee: |
Energy Research Corporation
(Danbury, CT)
|
Family
ID: |
22977552 |
Appl.
No.: |
06/257,740 |
Filed: |
April 27, 1981 |
Current U.S.
Class: |
429/478; 429/523;
429/528 |
Current CPC
Class: |
H01M
8/142 (20130101); H01M 2008/147 (20130101); Y02E
60/50 (20130101); Y02E 60/526 (20130101); H01M
2300/0051 (20130101) |
Current International
Class: |
H01M
8/14 (20060101); H01M 008/14 () |
Field of
Search: |
;429/35,33,34,41,44,16,112,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Skapars; Anthony
Claims
What I claim is:
1. An electrolyte-electrode assembly for use in a high-temperature
fuel cell comprising:
an anode electrode comprised of a first material;
a cathode electrode comprised of a second material;
an electrolyte member situated between said anode and cathode
electrodes and having an inner region and first and second regions
on opposite sides of said inner region facing said anode and
cathode electrodes, respectively, said first region having a
thermal coefficient of expansion between the coefficients of
thermal expansion of said anode electrode and said inner region and
said second region having a coefficient of thermal expansion
between the coefficients of thermal expansion of said cathode
electrode and said inner region.
2. An assembly in accordance with claim 1 wherein:
said first material is a metal;
said second material is one of a metal and metal oxide.
3. An assembly in accordance with claim 1 or 2 wherein:
said inner region and said first and second regions each comprise a
layer.
4. An assembly in accordance with claim 1 or 2 wherein:
said inner region includes an alkali-carbonate electrolyte
material.
5. An assembly in accordance with claim 4 wherein:
said inner region is non-conductive.
6. An assembly in accordance with claim 4 wherein:
said inner region further includes a first filler material for
inhibiting gas cross-over.
7. An assembly in accordance with claim 4 wherein:
one of said first and second regions includes a second filler
material for inhibiting gas cross-over.
8. An assembly in accordance with claim 7 wherein:
said first filler material and said second filler material are
substantially the same.
9. An assembly in accordance with claim 8 wherein:
said first and second filler materials are each comprised of
lithium aluminate.
10. An assembly in accordance with claim 4 wherein:
said first region includes said first material and said
alkali-carbonate electrolyte;
and said second region includes said second material and said
alkali-carbonate electrolyte.
11. An assembly in accordance with claim 10 wherein:
said inner region includes a filler material;
and one of said first and second regions includes said filler
material.
12. An assembly in accordance with claim 11 wherein:
said first and second regions each include said filler
material.
13. An assembly in accordance with claim 11 wherein:
said filler material concentration of said one of said first and
second regions is higher than the filler material concentration of
said inner region.
14. An assembly in accordance with claim 13 further comprising:
a third region between said one of said first and second regions
and said inner region, said third region having a coefficient of
thermal expansion between the coefficients of thermal expansion of
said one region and said inner region and including said filler
material of said one region said alkali-carbonate material, said
filler material in said third region being in concentration between
the concentration of filler material in said one and inner
regions.
15. An assembly in accordance with claim 14 wherein:
said one region is said first region.
16. An assembly in accordance with claims 12 wherein:
said first material is nickel;
and said second material is nickel oxide.
17. An assembly in accordance with claim 16 wherein:
said filler material is lithium aluminate.
18. An assembly in accordance with claim 17 wherein:
the total thickness of said regions containing said first material
is between 5-40 mils;
the total thickness of said regions containing said second material
is between 5-40 mils;
and the total thickness of the remaining regions is between 5-20
mils.
19. An assembly in accordance with claim 1 further comprising:
a first set of one or more regions disposed between said first
region and said anode electrode, the one of said regions of said
first set closest to said anode electrode having a coefficient of
thermal expansion which is between the coefficients of thermal
expansion of the immediately preceding region and said anode
electrode and each remaining region of said first set having a
coefficient of thermal expansion which is between the coefficients
of thermal expansion of the immediately preceding and immediately
succeeding regions.
20. An assembly in accordance with claim 1 or 19 further
comprising:
a second set of one or more regions disposed between said second
region and said cathode electrode, the one of said regions of said
second set closest said cathode electrode having a coefficient of
thermal expansion which is between the coefficients of thermal
expansion of the immediately preceding region and said cathode
electrode and each of said remaining regions of said second set
having a coefficient of thermal expansion between the coefficients
of thermal expansion of the immediately preceding and succeeding
regions.
Description
BACKGROUND OF THE INVENTION
This invention pertains to high temperature fuel cells and, in
particular, to electrode-electrolyte assemblies for use in such
cells.
Higher temperature fuel cells such as, for example, molten
carbonate cells, have the capability of producing electric power
from coal at system efficiencies approaching 50 percent. These
cells are thus an attractive candidate for alternative power
sources which conserve energy.
In the development of high temperature fuel cells to date, it is
customary to form the high temperature cells from discrete cathode
and anode electrodes which sandwich a discrete electrolyte tile.
The electrolyte-electrode assembly is then itself sandwiched
between cathode and anode gas housings to complete the cell. It is
also customary in this type of cell to add to the electrolyte tile
a binder or filler material to provide a mechanism for preventing
gas cross-over.
With the electrolyte-electrode assembly constructed in the above
manner, it is found that the fuel cell exhibits a certain degree of
contact resistance due to the lack of good contact over
substantially the entire areas of the stacked components. IR losses
also occur due to the necessary thickness of the electrolyte tile.
Limited fuel cell bubble pressure is also evidenced, owing to the
limited amounts of filler which can be added to the tile. Finally,
there is a tendency of the tile to crack during thermal cycling.
This cracking allows mixing of anode and cathode gases (gas
cross-over) which results in cell failure.
While the aforesaid cracking of the electrolyte tile is not fully
understood, it is generally believed to be due, at least in part,
to the rather different coefficients of thermal expansion of the
tile and electrodes. These differences and their effects are
further aggravated, by the discrete layer arrangement of the
assembly.
It is an object of the present invention to provide an electrolyte
electrode assembly for realizing an improved high temperature fuel
cell.
It is a further object of the present invention to provide an
electrolyte-electrode assembly for realizing a high temperature
fuel cell with increased power output and efficiency.
It is yet a further object of the present invention to provide an
electrode-electrolyte assembly with increased resistance to
cracking under thermal cycling.
It is also an object of the present invention to provide an
electrolyte-electrode assembly having increased bubble pressure and
reduced IR losses.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, the
above and other objects are realized in an electrolyte-electrode
assembly comprising an electrolyte member disposed between anode
and cathode electrodes, the electrolyte member being adapted to
have a first coefficient of thermal expansion in an interior or
inner region thereof and second and third coefficients of thermal
expansion in regions thereof on opposite sides of the inner region
and facing the anode and cathode electrodes, respectively, the
second coefficient of thermal expansion being between the first
coefficient of thermal expansion and the coefficient of thermal
expansion of the anode electrode and the third coefficient of
thermal expansion being between the first coefficient of thermal
expansion and the coefficient of thermal expansion of the cathode
electrode. Preferably, the inner region is formed as a layer and
contains electrolyte material. The other two regions are also
preferably layers and, if immediately adjacent the respective anode
and cathode, contain respective anode and cathode material and
electrolyte material.
With this type of construction for the electrolyte-electrode
assembly, there is a more gradual transition in the coefficient of
thermal expansion between each electrode and the electrolyte
member. In the preferred case mentioned above, this gradual
transition is due to the gradual change in element composition. It
results in better contact between the electrodes and the electrode
member and affords the member a greater resistance to cracking
during thermal cycling. Overall improved performance is thus to be
expected.
In further aspect of the invention, a layer of the electrolyte
member between the inner layer and one of the electrodes is
provided with filler in an amount sufficient to aid the assembly in
preventing gas cross-over, i.e., aids in maintaining the assembly
bubble pressure. This further enhances fuel cell performance. Also,
the inner layer can be made thinner reducing IR losses and adding
further to performance.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and aspects of the present invention
will become more apparent upon reading the following detailed
description in conjunction with the accompanying sole drawing which
shows in a schematic fashion a fuel cell incorporating an
electrolyte electrode assembly in accordance with the principles of
the present invention.
DETAILED DESCRIPTION
In the FIGURE, high temperature fuel cell 1 includes input
manifolds or housings 2 and 3 for coupling fuel process gas and
oxidant process gas to anode and cathode electrodes 4 and 5,
respectively. Disposed between these electrodes is an electrolyte
member 6 including an inner or interior electrolyte containing
region in the form of layer 6a. Typically, the high temperature
fuel cell 1 might be a molten carbonate cell in which the anode
electrode comprises a porous nickel material, the cathode electrode
a porous nickel oxide material and the inner electrolyte layer a
mixture of an alkali-carbonate and a filler or binder material for
enhancing the bubble pressure of the layer. Typical
alkali-carbonates might be potassium carbonate and lithium
carbonate, while a typical filler or binder might be, for example,
lithium aluminate.
In accordance with the invention, the electrolyte member 6 is
further constructed so as to exhibit improved contact resistance
and improved resistance to cracking during thermal cycling. More
particularly, the electrolyte member 6 is formed so as to have more
gradual transistion in thermal coefficient of expansion when
proceeding from each of the electrodes 4 and 5 to the inner region
6a. In the present illustrative case, this is achieved by
disposition of a first anode adjacent region in the form of a layer
6b between the anode electrode 4 and inner layer 6a, this layer 6b
having a coefficient of thermal expansion between the coefficients
of thermal expansion of the anode electrode 4 and the inner layer
6a. Similarly, a first cathode adjacent region in the form of a
layer 6c is disposed between the layer 6a and the cathode electrode
5. This layer 6c is formed to have a coefficient of thermal
expansion which is between the coefficients of thermal expansion of
the cathode electrode 5 and the inner layer 6a.
The desired coefficients of thermal expansion for the anode and
cathode adjacent layers 6b and 6c can be obtained by forming the
layer 6b from the anode electrode material and the material of the
inner layer in suitable proportion. Similarly, the layer 6c may
comprise suitable proportions of the cathode electrode material and
the inner layer material. For the case of nickel and nickel oxide
anode and cathode electrodes 4 and 5 and an alkali-carbonate-filler
electrolyte layer 6a, the layer 6b might comprise nickel and the
alkali-carbonate-filler electrolyte material and the layer 6c might
comprise nickel oxide plus such electrolyte material.
In accordance with a further aspect of the invention, one of the
layers 6b and 6c is further formed to fortify the bubble pressure
capability of the electrolyte member 6. Thus in the present
illustrative case, the layer 6b is provided with a filler
concentration increased over that of the layer 6a and such that the
anode electrode material of the layer is filled with the filler
material. The layer 6b thereby is made to possess a high bubble
pressure which, in turn, aids the bubble pressure already provided
to the member by the layer 6a.
In the present illustrative case, a further layer 6d is disposed
between the high-bubble pressure layer 6b and the inner layer 6a to
further ensure gradual transistion in thermal coefficients between
these layers. This further layer is formed from a mixture of
alkali-carbonate and filler, the alkali-carbonate content being
richer in this layer than the layer 6b and such that the layer
exhibits a coefficient of thermal expansion between those of the
layers 6b and 6a.
As can be appreciated, the number of additional layers situated
between each electrode and the inner electrolyte layer 6a to form
the electrolyte member 6 will depend upon the degree of gradual
transition in thermal coefficient desired. This, in turn, will
depend upon each particular application and the performance
characteristics attendant such application. In general, however,
each added layer preferably should have a material content which
results in a thermal coefficient of expansion which is at least
between those exhibited by its respective immediately preceding and
suceeding layers.
Construction of the electrolyte member in accordance with the
invention has the added advantage of enabling the predominant
electrolyte layers 6a and 6d, as well as the other electrolye
layers 6b and 6c, to be extremely thin. Thus, a total thickness for
the layers 6a and 6d as low as about 10 mils is possible, as
compared to a lower thickness of about 70 mils in prior structures.
This ability to obtain thin layers enhances electrolyte
conductivity as well as reduces thermal dimensional changes.
Greater power output and greater fuel cell efficiency can therefore
be realized with simultaneous realization of a more stable
electrolyte member.
A typical fuel cell constructed in accordance with the invention
might be as follows. The anode 3 might comprise a porous nickel
material of mean pore size of 2-12 microns upon whose surface is
impregnated lithium aluminate of particle size of 0.01 to 0.1
micrometers and concentration from 2 to 30 volume percent, this
impregnated layer then be filled with an alkali-carbonate in
concentration 20 to 60 weight percent based on the total weight of
alkali carbonate plus lithium aluminate to form the layer 6b. In
such case, the total thickness of the anode 3 and the layer 6b
might be 5-40 mils. The layers 6d and 6a might comprise a mixture
of lithium aluminate and alkali-carbonate in respective
concentrations of 30 to 70 weight percent for the layer 6d and of
20 to 60 weight percent alkali carbonate for the layer 6a. The
total thickness of these two layers might be in the range of 5-20
mils. Finally, the cathode electrode 5 might comprise nickel oxide
having a mean pore size of 3-20 microns, with the layer 6c being
formed on the surface of the electrode by deposition of the alkali
carbonate-lithium aluminate electrolyte composition of layer 6a.
These two elements, 6c and 5, might have a total thickness of 5-40
mils.
Formation of the electrodes 4 and 5 and the electrolyte member may
be accomplished using various techniques such as, for example,
spraying, electrophoretic deposition and/or filtration to obtain a
thin layered composite laminate. A particular process for producing
the assembly of the FIGURE would be to utilize a filtration and
melting process as follows. A porous anode electrode 4 is placed
into a filtering apparatus which is provided with a slurry of
lithium aluminate in a suitable working fluid. The slurry is
filtered under suction through the electrode structure to produce a
thin aluminate layer for formation of the layer 6b and a further
layer for formation of the layer 6d. Then, a controlled volume of
alkali-carbonate is placed on the outmost lithium aluminate layer
so that on raising the composite structure above the melting point
of the electrolyte under a CO.sub.2 containing non-oxidizing
atmosphere, the electrolyte melts and flows into the two lithium
aluminate layers filling them and rendering them non-porous,
whereby the formation of the layers 6b and 6d is brought to
completion. Electrolyte is prevented from flowing into the body of
the electrode 4 by the high bubble pressure layer 6b, which results
from the fine lithium aluminate particles of the layer entering the
pores of the nickel electrode. The layer 6a, in turn, is formed by
either filtration or electrophoretic deposition. The remainder of
the assembly i.e., the layer 6c is produced by a similar filtration
and melting process in the appropriate concentration of lithium
aluminate and alkali-carbonate on a porous nickel oxide electrode.
The two assemblies are then sandwiched together to form the
composite structure.
It would be equally feasible to start the process of building up
layers beginning with cathode. In this case either a porous nickel
or porous nickel oxide structure would be the starting cathode
electrode. If nickel were used it would be converted to nickel
oxide in the fuel cell during start up.
In all cases, it is understood that the above-described
arrangements are merely illustrative of the many possible specific
embodiments which represent applications of the present invention.
Numerous and varied other arrangements can readily be devised in
accordance with the principles of the present invention without
departing from the spirit and scope of the invention. Thus, another
configuration of layered electrolyte member 6 might be to include
two layers between each electrode and the inner electrolyte layer
6a. The layer closest the respective electrode might comprise the
electrode material into which has been imparted a concentration of
the alkali-carbonate material and the layer closest the inner
electrolyte layer might comprise alkali-carbonate material of
higher concentration than the electrode closest layer and electrode
material in lesser concentration than such electrode closest
layer.
* * * * *